Various chemical sterilizing agents, including chlorine dioxide, hydrogen peroxide and olefin oxides, such as ethylene oxide, are known. These sterilizing agents are used in the preparation of pharmaceutical and food products, in packaging, in manufacture of spices and in medical applications, for example. Chemical sterilants may be used in concentrated form, or may be diluted with an inert diluent, such as nitrogen, carbon dioxide or a chlorofluorocarbon. For example, dichlorodifluoromethane (CFC-12) has been widely used as a diluent for ethylene oxide, to maintain the ethylene oxide concentration in a sterilant gas mixture below the explosive threshold.
Ethylene oxide is highly toxic, and strict procedures must be followed in sterilization operations that use ethylene oxide, in quarantine of materials sterilized by exposure to ethylene oxide, and in disposal of exhaust gases from the sterilization operation. Several technologies for handling sterilizer off gases are available, including scrubbing, incineration, catalytic oxidation and other chemical reactions. U.S. Pat. No. 4,812,292 to L. Joslyn describes a scrubbing process in which an alkoxide sterilant gas is absorbed into an aqueous stream in a multistage absorption process. U.S. Pat. No. 4,828,810 to R. J. Kruse et al. describes a reaction process in which ethylene oxide in dilute concentrations is treated by exposure to a cation-exchange resin. Conventionally, the waste gas that is left after the sterilant has been removed to a safe level is vented to the atmosphere. However, scientific evidence linking CFCs and similar compounds to depletion of the ozone layer has made it imperative that they be removed from effluent streams. The United States and many other nations have signed an agreement entitled the "Montreal Protocol on Substances that Deplete the Ozone Layer". The Montreal Protocol calls for progressive production freezes on the following chemicals: CFC-11 (CCl.sub.3 F), CFC-12 (CCl.sub.2 F.sub.2), CFC-113 (C.sub.2 Cl.sub.3 F.sub.3), CFC-114 (C.sub.2 Cl.sub.2 F.sub.4), CFC-115 (C.sub.2 ClF.sub.5), Halon-1211 (CF.sub.2 ClBr), Halon-1301 (CF.sub.3 Br) and Halon-2402 (C.sub.2 F.sub.4 Br.sub.2). It is planned to reduce the production of CFCs to 50% of the 1986 levels in the next decade.
The sterilization industry is, therefore, faced with the need to find alternative sterilization procedures, to find alternative sterilant mixtures that do not contain CFCs, or to find alternative sterilizer exhaust treatment methods that reduce or eliminate atmospheric CFC emissions.
Alternative sterilization procedures include heat sterilization and radiation sterilization, both of which have been adopted in some situations. However, many pharmaceutical, food or medical products cannot withstand heat or radiation sterilization. At least as far as irradiation is concerned, public opinion may deprecate products that have been subject to irradiation, and FDA registration requirements may delay or preclude adoption.
Changing to sterilant mixtures that do not contain CFC diluents is not straightforward. Ethylene oxide forms potentially explosive mixtures with air over the range 3-100 vol % ethylene oxide. Pure ethylene oxide can be used as a sterilant, but demands explosion-proof equipment, stringent storage and handling precautions and brings other attendant problems. Even if nitrogen is used as a blanketing medium for the sterilization operation, equipment must be built and operated to withstand explosion. Ethylene oxide can be diluted with nitrogen or carbon dioxide, but to be non-flammable, the mixture must contain below 10 vol % ethylene oxide. Use of such dilute mixtures necessitates basic redesign of process equipment and operation cycles, and may again raise issues of compliance with FDA, OSHA, ASTM or other standards.
Limited attempts have been made to develop exhaust treatment methods that control diluent emissions. U.S. Pat. No. 4,831,196 to A. J. Buonicore et al. describes an exhaust gas treatment method aimed at first removing the olefin oxide sterilant from the exhaust gas, then subjecting the remaining exhaust gas to a compression-condensation operation to remove a portion of the inert diluent, before discharge to the atmosphere. Under the compression-condensation conditions described in the patent, it appears to be possible to recover about 60-80% of CFC-12, for example, in this way. Thus 20-40% of the inert gas used in the sterilant mixture is still emitted to the atmosphere. Applicants believe that no method to reduce atmospheric emissions below about 15% is presently available to the art.